Wireless communication systems are widely deployed in order to provide various kinds of communication services, such as voice and data. In general, the wireless communication system is a multiple access system which can support communication with multiple users by sharing available system resources (bandwidth, transmission power, etc.). Examples of the multiple access system include a CDMA (code division multiple access) system, an FDMA (frequency division multiple access) system, a TDMA (time division multiple access) system, an OFDMA (orthogonal frequency division multiple access) system, an SC-FDMA (single carrier frequency division multiple access) system and the like.
In a common wireless communication system, only one carrier is chiefly taken into consideration although the bandwidth between uplink and downlink is differently set. Hereafter, the carrier is defined by the center frequency and the bandwidth. The multiple carrier system uses a plurality of carriers having a bandwidth smaller than the entire bandwidth.
For example, a current 3GPP LTE (long term evolution) system based on the 3GPP (3rd Generation Partnership Project) TS (Technical Specification) Release 8 supports only one bandwidth (i.e., one carrier) of {1.4, 3.5. 10, 15, 20} MHz bandwidths. In order to support the total bandwidth of 40 MHz, the multiple carrier system uses two carriers each having a 20 MHz bandwidth or uses three carriers having a 20 MHz bandwidth, a 15 MHz bandwidth, and 5 MHz bandwidth, respectively.
The multiple carrier system is advantageous in that it can guarantee backward compatibility with the existing system and can greatly increase the data rate through multiple carriers.
3GPP LTE is based on dynamic scheduling in order to transmit and receive downlink data and uplink data.
In order to transmit downlink data, a base station first informs a user equipment of downlink resource allocation (this is called a downlink grant) through a control channel. The user equipment receives the downlink data through a downlink data channel indicated by the downlink resource allocation.
In order to transmit uplink data, a user equipment first transmits an uplink resource allocation request (this is called a scheduling request) to a base station. The base station which has received the uplink resource allocation request informs the user equipment of uplink resource allocation (this is called an uplink grant) through a control channel. The user equipment transmits the uplink data through an uplink data channel indicated by the uplink resource allocation.
There is a need for a scheme for performing dynamic scheduling more reliably in a multiple carrier system.